1. Field of the Invention
The present invention relates to a wet electrophotographic image forming apparatus and a method for controlling the same. More particularly, the present invention relates to a wet electrophotographic image forming apparatus and a method for controlling the same to individually control a development bias applied to a plurality of developing units.
2. Description of the Related Art
In general, a wet electrophotographic image forming apparatus is a printing apparatus for printing by using a liquid developing solution. The liquid development solution comprises a solidified particle toner for embodying colors and a liquefied carrier serving as solvent for solving the toner.
The wet electrophotographic image forming apparatus includes a light exposure unit for scanning laser beams according to printing data, a photosensitive medium on which electrostatic latent images are formed according to the laser beams emitted from the light exposure unit, a developing unit for developing the electrostatic latent images formed on the photosensitive medium, a transfer unit for transferring developed images to a printing medium, a fixation unit for fixing the images transferred onto the printing medium, a paper feeding unit staked with printing mediums and picking up each piece of paper from the stack of printing mediums, a printing medium conveying unit for conveying the printing medium, and a controller for controlling each said element to thereby conduct the printing process.
A wet electrophotographic image forming apparatus capable of printing in colors typically comprises four light exposure units and four developing units for developing four colors (ordinarily black, cyan, magenta and yellow).
Furthermore, the photosensitive medium includes four photosensitive drums corresponding to the four colors of developing units and a transferring belt for forming color images by allowing particular color images formed on the photosensitive drum to be transferred and overlapped, and for transferring the color images to the printing medium.
Now, a process of printing by the wet electrophotographic image forming apparatus will be explained in greater detail.
When the controller receives a printing command, the controller controls a light exposure unit so that laser beams corresponding to printing data received along with the printing command can be irradiated. The laser beams emitted from the light exposure unit form electrostatic latent images corresponding to the printing data on a surface of a photosensitive drum which is electrified to a predetermined voltage by a charging unit. When the photosensitive drum is rotated to allow a portion formed with the electrostatic latent images to be positioned opposite to the developing unit, the electrostatic latent images are developed in visible images by the developing solution of the developing unit. Now, a process of the electrostatic latent images on the photosensitive drum being developed by the developing unit will be explained in further detail.
Two bias voltages are used to move the developing solution from a developing solution deposit roller to a developing roller, and then to a photosensitive drum. First a developing solution supply bias is applied to a developing solution deposit roller for supplying the developing solution loaded in a developing solution container to a developing roller in order to help move the developing solution attached to the surface of the developing solution deposit roller to the surface of the developing roller. Next, the developing solution attached to the surface of the developing roller is moved to the surface of the photosensitive drum by a development bias to develop the electrostatic latent images to form the visible images.
When printing in full color, electrostatic latent images corresponding to each color are formed on the four photosensitive drums, and respective electrostatic latent images are developed in images of particular colors by the developing units corresponding to each photosensitive drum.
The visible images developed in particular colors on the plurality of photosensitive drums are transferred to and overlapped on the transferring belt to form images in full color. The images in full color formed on the transferring belt are transferred to the printing medium by the transfer unit. The images formed on the printing medium are fixed on the printing medium by the fixation unit. The printing medium having completed the fixation is discharged outside of a body of the image forming apparatus by the printing medium conveying unit.
Wet electrophotographic printing processes use a wet toner made up of a liquid carrier and solid toner particles. The carrier is used to move the toner to the electrostatic latent images of the photosensitive medium, and the toner particles form the color images. However, if there is a shortage of carrier liquid, the density of the developing solution increases to make the developing solution sludgy, resulting in problems such as insufficient circulation of the developing solution, among others. As a result, for the best performance it is necessary to maintain the density of the developing solution at a substantially constant level.
In order to maintain the density of the developing solution at a constant level, it is necessary to measure the density of the developing solution and to supply additional carrier as needed. There are two methods of replenishing the carrier for maintaining the density of the developing solution at a constant level. One method is to use new carrier and the other method is to reuse carrier by retrieving and condensing carrier liquid that was evaporated during the printing process.
However, there is a problem in the former method in that new carrier needs to be continuously supplied, causing an over-consumption of carrier. There is also a problem in the latter method in that a retrieval apparatus is needed for retrieving and condensing the evaporated carrier, adding cost and complexity to the printing apparatus.
Accordingly, there is a need for a wet electrophotographic printing apparatus which maintains a developing solution density at a constant level while minimizing the use of new carrier while avoiding the size and complexity of conventional carrier retrieval apparatuses.